1. Field of the Invention
The present invention relates in general to a drive circuit for a power semiconductor device. In particular, the invention relates to a drive circuit for driving a power semiconductor device such as an IGBT (Insulated-GATE Bipolar Transistor), the drive circuit serving to suppress a surge voltage which is generated when carrying out the switching operation.
2. Description of the Related Art
A description will hereinafter be given with respect to a method or protecting an IGBT from an over-current in a conventional power semiconductor device. When an over-current is caused to flow through the IGBT, a gate potential of the IGBT and a reference voltage are compared with each other by a comparator. Then, only when a gate-emitter voltage of the IGBT is increased beyond the reference voltage, the comparator operates to turn ON a switch. Thus, a gate voltage is clamped to a breakdown voltage of a Zener diode and a breakdown voltage of a diode so that a period of time up to the breakdown of the IGBT during the flow of the over-current is prolonged. By the way, even if the gate-emitter voltage is decreased, the switch which has been turned OFF once by the mono-multi operation is not turned ON again (e.g., refer to JP 4-165916A, p. 2 (Patent Document 1)).
In addition, a description will now be given with respect to one example of a drive circuits for a power semiconductor device having a protective device utilizing the increase in gate-emitter voltage during the flow of an over-current in another conventional device. The conventional device is an over-current protective device for a power device which includes a photocoupler having an input side provided between a gate of a power device provided in a load circuit and a positive electrode of a D.C. power source for gate drive in order to restrict an increase in gate voltage of the power device due to the flow of an over-current and to insulate a current on the input side due to the over-current so as to be able to output an output signal, and which serves to cut off or restrict the over-current flowing through the power device by using the output signal from the photocoupler as a detection signal for the over-current. In addition, the photocoupler includes an LED on the input side and a phototransistor on the output side (e.g., refer to JP 2614355 A (p. 3, FIG. 1 (Parent Document 2)).
In the conventional device having such a configuration, as the gate voltage is increased during the flow of an over-current, it causes the LED to conduct, and the phototransistor outputs to a control circuit a signal of the current corresponding to the conduction current through the LED, as a detection signal for an over-current. Then, the control circuit outputs a control signal so as to cut off or restrict the conduction of the power semiconductor device on the basis of the detection signal inputted thereto. In addition, the LED also serves the effect of clamping an increase in gate voltage Vge to suppress peak current of an over-current.
In the over-current detection system utilizing the increase in gate-emitter voltage typified by the above-mentioned Patent Documents 1 and 2, the gate-emitter voltage needs to be increased to some degree, and the current value at which the over-current can be detected often reaches a value which is several times as large as that of the rated current as the case may be. For this reason, when an OFF-command is issued from the outside in the state in which there is flowing a current having a level below that of the current which can be detected as the over-current even if the current is equal to or higher than the rated current of the power semiconductor device, the current caused to flow through the power semiconductor device is cut of normally. There is, as a result, the possibility that a surge voltage which is generated in cutting off a current is increased to cause destruction of the device. In particular, in the case where the current increase rate di/dt is low when short-circuit occurs in a place remote from a power semiconductor device, and so forth, there is encountered the problem that since it takes a time until the gate-emitter voltage reaches the detection level, the probability that an OFF-command is issued from the outside during that period of time is increased, thereby increasing the possibility that the device is destroyed.